1. Technical Field
This invention relates generally to the field of printed circuit board assembly, and more specifically, to a method and apparatus for facilitating the calibration of pick and place equipment.
2. Background of the Invention
Integrated and discrete components are commonly assembled onto printed circuit boards (PCBs) using pick and place equipment to set the various component parts on the board prior to attachment. Fuji America Corporation of Lincolnshire, Ill., as well as other manufacturers provide a variety of pick and place equipment. The present invention may be used in calibrating the "Z" origin position of a part placement nozzle relative to the height of the assembly table or some other preselected reference elevation.
Pick and place devices typically include a conveyor section which receives and transports a PCB onto an assembly table. The table is capable of movement in commonly two and occasionally in three axes. The PCB is positioned generally below the placing head.
Each placing head includes at least one part placement nozzle, which picks up a component part by employing typically a vacuum applied to the upper face of the component part. A variety of configurations exist for pick and place equipment, including multiple placing head configurations and multiple placement nozzle configurations.
In operation, a program, typically run by a processing device contained within or otherwise connected to the pick and place device, contains a series of commands which direct the pick and place device to select a nozzle by rotating a carousel head to a preselected position as well as by rotating a multiple nozzle holder to a preselected position. The program next directs the pick and place device to pick a component part from typically a number of feeder stations which dispense component parts, for example from a tape-and-reel which dispenses a particular component part at a preselected position. After the part placement nozzle has picked the component part, the placing head rotates as required to position the component part over the PCB. The assembly table is also advanced along its "X" and "Y" axes as required to position the PCB for placement of the component part. As positioning occurs, the component part is also aligned with the PCB by rotation of the placing head to orient the component part as required relative to the PCB.
The device next places the component part against the upper surface of the PCB in its preselected location, the part placement nozzle typically exerting pressure against the upper face of the component part, securing the component part against the face of the PCB. In a subsequent process step, the component part is permanently affixed to the surface of the PCB, often by heating a solder which has been previously applied to the surface of the PCB.
In a number of pick and place devices, both the carousel head and the assembly table are capable of travel in the "Z" axis. In these devices, calibration of the "Z" origin position is critical for operation, as parts which are placed with excessive pressure may be damaged and a placement employing insufficient pressure may be flawed as well. In order to calibrate a "Z" origin position for operation, a technician or other operator "locks-out" the device employing typically a key or set of keys which are inserted into a slot or slots bypassing the front safety doors of the machine. Once bypassed, the operator extends one or both arms into the machine setting the magnetic base of a dial gauge against a magnetic surface for attachment. The carousel head and the nozzle holder are both rotated to preselected positions in order to select the calibration nozzle. The operator adjusts the feeler tip of the gauge to contact the nozzle so that the feeler gauge will be responsive to the vertical travel of the nozzle. The assembly table which, for the purpose of calibration holds a calibration plate, is raised along its "Z" axis to a position where the nozzle is just touching the calibration plate. Calibration is presently achieved by setting a dial gauge within the machine's housing in a position which allows a feeler tip on the gauge to contact the nozzle for calibration. This initial position is recorded as the zero or reference point. The assembly table is raised a specified distance exerting an upward pressure against the nozzle, typically on the order of a fraction of a millimeter as measured on the dial gauge. This position is set as the "Z" origin position. The calibration tooling is removed from the machine and the machine is placed back in service employing the recently set "Z" origin position as a reference value for the downward travel of the placing head and the nozzle during operations.
It has been observed that this method is somewhat cumbersome as it requires a technician or other operator to first "lock-out" the device. Once bypassed, the operator extends one or both arms into the machine setting the dial gauge base against a magnetic surface for attachment. Next, the operator adjusts the feeler tip of the gauge to extend below the nozzle. This method exposes the operator's arms and hands to the possibility of injury within the machine. Additionally, because the dial gauge is placed manually within the machine, the assurance of test repeatability decreases. Additionally, it is difficult for the operator to assure proper alignment of the feeler tip of the gauge below the nozzle. What is needed is a method and apparatus which allows for the safe and accurate placement of the dial gauge below the part placement nozzle.